FIG. 1 is an energy level diagram of a thulium ion (see, non-patent document 1). In FIG. 1, energy values are shown on the right side of individual energy levels, and names of the individual levels are shown on the left side of the individual energy levels. Numerals added to arrows indicate wavelengths of light lasored (corresponding to upward arrows (not shown in FIG. 1)) or emitted (corresponding to downward arrows in FIG. 1) when transitions of the individual arrows occur. Here, the unit of the energy is represented by 1/cm (corresponding to Kayser in terms of spectroscopy) based on the unit of the wave number, and the name of the energy levels are based on the Russell-Saunders notational system. In addition, alphabetical capitlas represent a compound orbit angular momentum, superscript index digits added to them represent the multiplicity of the spectra term based on electronic total spin angular momentum, and subscript index digits added to them represent the total angular momentum. Here, each level is the level having an expanded width because of the segmentation of degeneration levels by the Starke effect caused by crystal electric field.
As for a fiber having its core doped with thulium (Tm), applications to fiber lasers, spontaneous emission sources, or optical fiber amplifiers have been studied employing the following bands of the thulium ion in FIG. 1:                1.9 μm band using 3H4→3H6 transition (which represents the transition of the thulium ion energy from the 3H4 level to the 3H6 level. This notational system will be used from now on);        2.3 μm band using 3F4→3H5 transition;        0.82 μm band using 3F4→3H6 transition; and        1.48 μm band using 3F4→3H4 transition.Incidentally, to implement the fiber lasers, spontaneous emission sources, or optical fiber amplifiers at a high efficiency between the transitions above-mentioned, fluoride fibers are used as fibers to which Tm (thulium) is added. Among the Tm-doped fluoride fibers, the 2.3 μm band, in particular, is difficult to oscillate by semiconductor lasers, has hidden potential to become a huge business, and attracts great attention as a light source for noninvasive blood glucose level sensing which many foreign and domestic medical inspection instrument developers compete fiercely to develop.        
Up to now, the following have been reported:    (1) Laser oscillation at 0.82 μm band, 1.48 μm band, 1.9 μm band and 2.35 μm band implemented by applying 0.67 μm band excitation (excitation of the thulium ions at the 3H6 level to the 3F3 level) to the Tm-doped fluoride fiber (see, non-patent document 1);    (2) Laser oscillation at 2.35 μm band implemented by applying 0.8 μm band excitation (excitation of the thulium ions from the 3H6 level to the 3F4 level) to the Tm-doped fluoride fiber (see, non-patent document 2 or patent document 1);    (3) Laser oscillation at 0.82 μm band, 1.48 μm band, 1.9 μm band and 2.35 μm band implemented by applying 0.8 (0.79) μm band excitation to the Tm-doped fluoride fiber (see, patent document 1);    (4) Laser oscillation and an optical fiber amplifier at 1.9 μm band implemented by applying 1.55–1.75 μm band excitation, excitation of the thulium ions from the 3H6level to the 3H4 level, to the Tm-doped fluoride fiber (see, patent document 2); and    (5) Laser oscillation and an optical fiber amplifier at 1.48 μm band implemented by applying 1.06 μm band excitation to the Tm-doped fluoride fiber (see, patent document 2).
The 2.3 μm band fiber lasers have already been developed as described in the foregoing reports (1), (2) and (3).
Patent Document 1: Japanese Patent Application Laid-open No. 3-293788 (1991);
Patent Document 2: Japanese Patent Application Laid-open No. 6-283798 (1994);
Non-Patent Document 1: J. Y. Allain et al., “Tunable CW lasing around 0.82, 1.48, 1.88 and 2.35 μm in Thulium-doped fluorozirconate fiber” Electron. Lett., Vol. 25, No. 24, pp. 1660–1662, 1989;
Non-Patent Document 2: L. Esterowitz et al., “Pulsed laser emission at 2.3 μm in a Thulium-doped florozirconate fiber”, Electron. Lett., Vol. 24, No. 17, p. 1104, 1988;
Non-Patent Document 3: A. Taniguchi, et al., “1212-nm pumping of 2 μm Tm-Ho-codoped silica fiber laser”, Appl. Phys. Lett., Vol. 81, No. 20, pp. 3723–3725, 2002; and
Non-Patent Document 4: P. R. Barber, et al., “Infrared-induced photodarkening in Tm-doped fluoride fiber”, Opt. Lett., Vol. 20 (21), pp. 2195–2197, 1995.